The invention relates to a photopolymerizable mixture containing a polymer binder, a polymerizable compound, in particular an acrylic or alkacrylic ester, and a photoinitiator combination.
Photopolymerizable mixtures containing certain combinations of photoinitiators and activators, for example carbonyl-containing initiators and tertiary amines, for increasing the light sensitivity are known. Mixtures of this type which have a synergistic action are described, for example, in U.S. Pat. Nos. 3,759,807, 4,054,682 and 4,071,424. A disadvantage of these mixtures containing low-molecular-weight amines is that they have low stability on storage, since the amines tend to exude easily, in particular from thin films.
JP-A 50/129,214 (applied for on Apr. 2, 1974 under the number 49/36,614) describes a photopolymerizable mixture containing a tetra(meth)acrylic ester of an N,N,N',N'-tetrahydroxyalkylalkylenediamine as the polymerizable compound. The tetrafunctional compound serves as crosslinking agent.
Furthermore, it is known to initiate the free-radical polymerization of ethylenically unsaturated compounds by irradiation with visible light in the presence of photoreducible dyes and reducing agents, for example amines (U.S. Pat. No. 3,097,096). However, these initiator combinations are in most cases only used in aqueous solution or in a combination with water-soluble binders. Initiator combinations comprising photoreducible dyes and other reducing agents are described in U.S. Pat. Nos. 3,597,343 and 3,488,269.
Photopolymerizable mixtures containing exclusively photoreducible dyes as initiators have so far not been used in practice, due to their low light sensitivity. JP-A 54/151,024 describes a photopolymerizable mixture containing an initiator combination comprising a merocyanine dye and a trihalomethyl-substituted s-triazine, which is sensitive to visible light, for example an argon laser. However, the sensitivity of these mixtures to visible laser light is not sufficient for industrial applications.
EP-A 287,817 describes photopolymerizable mixtures containing (meth)acrylic esters having urethane groups, tertiary amino groups and possible urea groups in the molecule, polymer binders and, as photoinitiators, a combination comprising a photoreducible dye, a radiation-sensitive trihalomethyl compound and an acridine, phenazine or quinoxaline compound.
EP-A 321,826 describes a similar mixture containing (meth)acrylic esters but no urethane groups.
U.S. Pat. No. 3,717,558 describes metallocenes of elements of subgroups IIa to VIIIa in combination with a further photoinitiator containing a carbonyl chloride group for use in photopolymerizable recording materials. These initiator combinations are very sensitive to oxygen and hydrolysis and therefore less suitable for producing printing plates and resist materials. Further metallocenes and their use as photoinitiators in photopolymerizable mixtures are described in EP-A 119,162 and 122,223. These are titanocenes having good resistance to air and exhibiting spectral sensitivity in the range from UV light to visible light. They contain, inter alia, cyclopentadienyl radicals and fluorinated phenyl radicals as ligands. Furthermore, EP-A 242,330 and 269,573 describe photopolymerizable mixtures containing a photoinitiator mixture comprising a titanocene and a liquid photoinitiator of the hydroxy- or aminoacetophenone type. In these materials, the obtainable light sensitivity is not sufficient for rapid imaging using a low-energy and, at the same time, low-cost argon ion laser.
The older European Patent Application 89117004.5, which is not a prior publication, describes photopolymerizable mixtures containing: a polymer binder; a compound which is polymerizable by free radicals and contains at least one polymerizable group; a photoreducible dye; a trihalomethyl compound which can be cleaved by radiation; and a metallocene compound, in particular a titanocene or zirconocene.
The metallocenes used are those carrying two substituted or unsubstituted cyclopentadienyl radicals and two substituted phenyl radicals as ligands. The phenyl radicals carry at least one fluorine atom in the o position relative to the bond and contain, if desired, further substituents, such as Cl, Br, alkyl or alkoxy groups; the compounds which are preferred and described in the examples are those containing pentafluorophenyl radicals. These mixtures have extremely high light sensitivity. However, their stability on storage is limited, in particular at elevated temperatures.